The present invention relates to a coaxial pipe assembly including a thermally insulating sleeve, and more particularly a sleeve for insulating the connection zone between two coaxial pipe strings, in particular undersea pipes for conveying hot and cold fluids, and preferably undersea pipes for use in great depths.
In most industrial fields, high performance insulation systems are sought after in order to maintain the fluids that are conveyed by pipework at constant temperature so that transfer between pieces of equipment can be made possible over considerable distances, e.g. reaching as much as several hundreds of meters, or even several kilometers. Such distances are commonplace in industries such as oil refineries, liquefied natural gas installations (at −165° C.), and undersea oil fields that extend over several tens of kilometers. Such oil fields are being developed in ever-increasing depths of water, which depths may exceed 3000 meters (m).
The present invention relates in particular to coaxial pipe elements for fabricating undersea pipes installed over oil wells at great depths, in particular bottom-to-surface connection pipes in suspension between a surface ship anchored on said oil field and the sea bottom.
Such coaxial pipes are referred to as pipe-in-pipe (PiP) assemblies, with an inner pipe serving to convey the fluid and an outer pipe arranged coaxially around the inner pipe, also referred to as the “outer casing”, that comes into contact with the surrounding medium, i.e. sea water. The annular space between the two pipes may be filled with an insulating material, or all gas may be evacuated therefrom.
Such systems have been developed to achieve high levels of thermal performance and specific versions have been developed to be better adapted to great depths, i.e. in order to withstand pressure at the sea bottom. Water pressure is substantially 0.1 megapascals (MPa) i.e. about 1 bar, for every 10 m of depth, so the pressure that needs to be withstood by the pipe is about 10 MPa, i.e. about 100 bars, at a depth of 1000 m, and about 30 MPa, i.e. about 300 bars, at 3000 m.
Such coaxial pipe assemblies are fabricated by butt-welding together unit lengths referred to below as “coaxial pipe unit elements” or by assembling together “coaxial pipe strings”, where each string is itself made up of a plurality of pipe unit elements, and generally has a length lying in the range 10 m to 100 m, more particularly each presenting a length of 12 m, or 24 m, or 48 m.
In the context of installing undersea pipes at great depths, these unit length elements or these strings are fabricated on land. They are then transported out to sea on a laying ship. While they are being laid, the coaxial pipe assembly strings or unit elements are connected to one another on board the ship while they are being laid at sea. It is therefore important that such connection can be integrated in the method of constructing and assembling the pipe and laying in on the sea bottom.
To do this, it is general practice to use “junction pieces” or “connection forgings” that are made of steel and that are assembled to the ends of said coaxial pipe assembly elements for assembling together. The junction piece at the downstream end of a first coaxial pipe assembly element that has not yet been assembled is connected by welding to the free junction piece at the upstream end of a second coaxial pipe assembly element that has already been assembled at its downstream end.
These junction pieces also serve to reinforce the strength of pipes subjected to high levels of bending during laying, in particular in connection zones between two said successive unit lengths, and more particularly with bottom-to-surface connections, in order to give them very great resistance to fatigue throughout the lifetime of the installations.
More particularly, said junction pieces comprise two branches in the form of bodies of revolution, an outer branch and an inner branch together forming a fork defining said annular space, the free cylindrical ends of the fork being assembled directly to the cylindrical ends respectively of the outer and inner pipes. Coaxial pipes and junction pieces or pieces of this type are described in particular in FR 2 873 427 and GB 2 161 565.
FR 2 786 713 and FR 2 897 919 describe another embodiment in which a junction piece is not used to close the annular space between the inner and outer pipes at their ends, but rather the ends of the inner pipe are caused to project relative to the ends of the outer pipe and a terminal portion of the outer pipe is deformed around the terminal portion of the inner pipe by constricting its diameter until the two pipes come close together so as to close the annular space, in particular by welding together the ends of the outer and inner pipes. This type of closure of the annular space and of junction between the ends of the coaxial pipes is referred to as “crimping”. It is advantageous since it allows a welding machine to have access to the ends of the inner pipes of both successive coaxial pipe elements that are to be assembled together in order to butt-weld them without being impeded by the associated outer pipes. The space between the ends of the two outer pipes of the two coaxial pipe elements assembled end-to-end is generally covered by a tubular sleeve providing insulation and mechanical reinforcement to the junction, in particular a sleeve that slides over said outer pipe.
The object of the present invention is thus to provide PiP type coaxial pipes, in which the pipe strings or unit elements present over their entire length, apart from at their ends, a level of insulation that is extreme because of the presence of an insulating material in the annular space between their inner and outer pipes, preferably associated with pumping out a high vacuum so as to limit phenomena of heat being transmitted by convection. In contrast, at the ends of said strings, the solid metal connection between the outer pipe and the inner pipe, whether via a junction piece or by crimping, eliminates thermal insulation between the ambient medium, generally water at 3° C.-5° C. at very great depth, and the oil being conveyed, generally at a temperature lying in the range 40° C.-45° C. to 80° C.-100° C., or even higher. It therefore follows that a particularly large amount of heat transfer takes place via said junction zone, whether provided by welding together two said junction pieces or via “crimped” coaxial pipe unit elements, with a consequent loss of heat from the oil being conveyed, thereby running the risk of giving rise to plugs of paraffin or to the formation of gas hydrates, should the temperature drop below 30° C.-35° C. at any point along the bottom-to-surface connection. Thus, it is desirable locally to restore a level of insulation at each of the junctions between strings that is sufficient to enable oil to be conveyed between the well heads and the surface with minimum loss of heat.
This thermal bridge problem is even more critical in the event of production being stopped, since the crude oil column is then stationary and although heat losses from the main portion of the string remain low, given the extreme level of insulation due to the PiP principle, the same does not apply at the connections between strings because of the thermal bridges created thereby. A PiP string having a unit length of 48 m presents a level of insulation over its main portion that is better than 1 watt per square meter per kelvin (W/m2/K), with its mean value being degraded by 10%-12% as a result of the thermal bridge existing at each of its ends, when said ends are not insulated. If the unit length is half that, i.e. 24 m, then the degradation due to non-insulated ends is doubled, i.e. 20%-25%. Similarly, for strings having a length of 12 m, the degradation can be as much as 40%-50%, thereby depriving PiP technology of any advantage, given the great complexity and cost involved in fabricating it.
The term “mean degradation value” is used herein to mean the overall loss of heat from the length of a string, including the junction pieces or the crimped zone at each of its two ends, divided by the length of said string.
External pipe insulation means are known that withstand high hydrostatic pressures and that are therefore suitable for being used at great depths, said means being constituted by:                coatings of quasi-impressible solid polymer materials based on polyurethane, polyethylene, polypropylene, etc., that, where appropriate, are in the form of a solid tubular sleeve. However such materials present fairly mediocre thermal conductivity and thermal insulation properties, i.e. properties that are not sufficient to overcome the drawbacks in the event of production being stopped in undersea pipes conveying hydrocarbons; or        coatings of synthetic materials constituted by hollow beads containing gas and capable of withstanding external pressure, the beads being embedded in binders such as concrete, epoxy resin, etc. . . . presenting thermal insulation properties that are better, but that are considerably more expensive and more difficult to install. In practice, recourse is made to half-shells that are assembled around the welded junction that needs to be protected after the welding has been performed.        
Furthermore, insulating materials are known that present superior thermal insulation properties, i.e. lower thermal conductivity, and in some cases associated with phase-change properties, in particular materials in gel form.
Materials of this type are described in particular in the following patents: FR 2 800 915; FR 2 820 426; FR 2 820 752; WO 2004/003424; and WO 00/40886. Nevertheless, because of their insufficient mechanical strength and because they are obtained by physical and chemical or physicochemical reactions between a plurality of components, such insulating gels need to be injected in liquid form immediately after their various components have been mixed together and inserted or injected between the outer and inner pipes of a PiP type coaxial pipe assembly.
The mechanical strength of such a gel is insufficient on its own for withstanding the mechanical stresses to which pipes resting on the sea bottom are subjected, so they are not implemented on the outsides of the outer pipes of PiP pipe assemblies. That is why they are not suitable for insulating junction zones between two PiP pipe sections assembled together by butt-welding, possibly via junction pieces constituting bodies of revolution.
Document WO 2008/053251 describes various means for insulating a thermally insulating sleeve for the junction zone between two pipe unit elements, said thermally insulating sleeve being constituted by PiP pipes of steel connected together at their ends by crimping, with the annular space thereof containing a thermally insulating material.
In WO 2008/053251, in its FIGS. 3 and 4, the inside volume between the two walls of the sleeve is partially filled with an insulating material (see page 13, lines 14 to 22) and partially filled with a gas such as air.
In that document, on page 13, line 20, it is stated that the steel PiP structure provided the insulation for the junction zone is expensive and heavy. That is why, that document proposes an improvement using an insulation system as described in FIG. 5, in which the sleeve is replaced by a lighter casing together with a single steel pipe element (see page 18, lines 26 et seq.), which element is covered in multilayer insulation (see page 19, lines 5 et seq.) of expanded polypropylene foam or polyurethane syntactic foam.
However, such a steel sleeve is counterproductive in terms of thermal insulation, given the high thermal conductivity of steel.